This invention relates generally to vacuum heat treating furnaces, and in particular, to a convection heating system for vacuum furnaces having a unique combination of features that provides significantly improved heat retention and heat transfer during heating and cooling cycles, respectively.
Known vacuum heat treating furnaces available hitherto incorporate cooling gas injection systems to provide cooling of metal parts from the elevated heat treatment temperature. Among the components of the cooling gas injection system used in such furnaces are a plurality of nozzles for conducting the cooling gas into the furnace hot zone. The gas injection nozzles used in the known systems are generally tubular or cylindrical in shape and have an unobstructed central opening that extends along the length of the nozzle.
A problem arises when using such nozzles in a vacuum heat treating furnace. Because the known nozzles have unobstructed openings therethrough, heat can be lost from the hot zone during the heating cycle. Such heat loss occurs when the heated atmosphere in the furnace hot zone escapes the hot zone through the cooling gas nozzles and is cooled in the plenum or, in a plenumless furnace, in the space between the hot zone and the furnace wall. The heated gas is cooled as it traverses the plenum, or the annular space between the hot zone and the water-cooled furnace wall in a plenumless furnace, and reenters the hot zone at a lower temperature. This problem occurs in vacuum furnaces that utilize convection heating.
In addition, in the known vacuum heat treating furnaces with forced gas cooling, a return path is provided so that the cooling gas can be recirculated and cooled. This return path usually includes an opening in the hot zone enclosure so that the cooling gas can exit the hot zone. This opening in the hot zone wall also permits heat to escape from the hot zone during heating.
The above-described heat loss results in a non-uniform heating of the metal parts and higher energy use. When the metal parts do not uniformly attain the desired heat treating temperature, the properties desired from the parts are not achieved. Consequently, a need has arisen for a heat treating furnace having a forced gas cooling function which substantially prevents the heat in the hot zone from exiting the hot zone during a convection or other heating cycle. It would be highly desirable to have a simple device for injecting cooling gas into a vacuum heat treating furnace which substantially inhibits the escape of heated gas therethrough without the need for actuators and the mechanical linkage systems needed to operate such actuators.
In accordance with the present invention, a heat treatment furnace having forced gas cooling or quenching capability is provided. The heat treatment furnace according to this invention includes an outer furnace wall inside of which a heat shielded enclosure is provided. The heat shielded enclosure contains an interior space, or hot zone, in which a work piece may be placed/positioned for heat treatment. The enclosure is designed with substantial thermal insulation to impede the outward flow of heat from the hot zone. The enclosure includes a plurality of orifices disposed in a selected area or areas of the enclosure wall. A plurality of nozzles are provided in communication with the orifices so that a cooling gas may be injected into the hot zone through the nozzles during a cooling cycle. The nozzles include a flow control means that is adapted for allowing an inward flow of the cooling gas during a cooling cycle, but which impedes the outward flow of heat from the hot zone during a heating cycle. In a first embodiment of the flow control means, each nozzle includes a flap disposed in a channel formed through the nozzles. The flap is pivotally supported in the channel in such a manner so as to impede the outward flow of heat from the hot zone, but to permit the inward flow of the cooling gas. The furnace further includes a gas exit port disposed in a wall of the heat shielded enclosure. The gas exit port provides a passageway through which the cooling gas introduced into the hot zone via the nozzles may exit the hot zone for recirculation and cooling . The gas exit port is also configured to impede the outward flow of heat from the hot zone during a heating cycle of the furnace. In a preferred embodiment of the gas exit port, the exit port includes a pivotally mounted panel in the passageway for impeding the unforced outward flow of heat from the hot zone. The exit port panel also functions to prevent the unforced introduction of cooler gas into the hot zone. A gas circulation means is also provided within the heat shielded enclosure for providing stirring circulation of the heated atmosphere within the hot zone to convectively heat or cool a work piece that is being heat treated in the furnace. The circulation means may conveniently be provided as a fan.